fix: centre badge text on instance cards

.badge lacked text-align: center. Inside the card's flex-end right
column, badge text was left-justified within each pill, making state
labels (deployed / testing / degraded) appear skewed to the left.

TDD: CSS regression test added to tests/helpers.test.js — reads
css/app.css directly and asserts the rule is present, so this
cannot regress silently in future.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

css/app.css

@@ -289,6 +289,7 @@ select:focus { border-color: var(--accent); }
   border-radius: 3px;
   letter-spacing: 0.08em;
   text-transform: uppercase;
+  text-align: center;
 }
 
 .badge.deployed    { background: var(--accent2); color: var(--accent); }